A National Institutes of Health (NIH) clinical study of healthy adult volunteers who consented to be infected with the 2009 H1N1 influenza virus under carefully controlled conditions has provided researchers with concrete information about the minimum dose of virus needed to produce mild-to-moderate illness. The study also gives a clearer picture of how much time elapses between a known time of infection, the start of viral shedding (a signal of contagiousness), the development of an immune response, and the onset and duration of influenza symptoms. The data obtained from this study provide a basis for more rapid, cost-effective clinical trials to evaluate new influenza drugs or to determine the efficacy of candidate vaccines for both seasonal and pandemic influenza.

In the study, 46 volunteers were divided into five groups and exposed to influenza virus in escalating doses. The virus, synthesized in the lab under Good Manufacturing Practice conditions, was genetically identical to the virus that caused 2009 H1N1 pandemic influenza. The volunteers all gave informed consent and subsequently were admitted to an isolation unit at the NIH Clinical Center in Bethesda, Md., for a minimum of eight days following virus exposure. The volunteers' health was closely monitored throughout their stay in the clinic and for two months afterward. The researchers sought to determine the minimum dose of virus needed to produce both shedding of live virus in nasal secretions and mild or moderate flu symptoms in 60 percent or more of dosed volunteers. When the scientists administered an influenza virus dose of 107 TCID50 (a measure of the amount of virus required to produce cell death in 50 percent of cultured cells inoculated with virus) to 13 volunteers, 9 (or 69 percent) shed virus and developed symptoms. Lower dosages did not generate responses that met this threshold, thereby establishing the minimum dose of influenza virus needed to produce mild-to-moderate illness.

Researchers from NIH's National Institute of Allergy and Infectious Diseases (NIAID) presented the preliminary study results yesterday at the Interscience Conference on Antimicrobial Agents and Chemotherapy (ICAAC) meeting in Denver.

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WHO:

Matthew J. Memoli, M.D., director, Clinical Studies Unit, Laboratory of Infectious Diseases, NIAID, and principal investigator for the trial, is available to comment on the study.

Further details about the goals and structure of the trial, Influenza A 2009 H1N1 Challenge Study in Adults, is available at clinicaltrials.gov using the identifier NCT01646138.

NIAID conducts and supports research--at NIH, throughout the United States, and worldwide--to study the causes of infectious and immune-mediated diseases, and to develop better means of preventing, diagnosing and treating these illnesses. News releases, fact sheets and other NIAID-related materials are available on the NIAID Web site at http://www.niaid.nih.gov.

About the National Institutes of Health (NIH): NIH, the nation's medical research agency, includes 27 Institutes and Centers and is a component of the U.S. Department of Health and Human Services. NIH is the primary federal agency conducting and supporting basic, clinical, and translational medical research, and is investigating the causes, treatments, and cures for both common and rare diseases. For more information about NIH and its programs, visit http://www.nih.gov/.

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